There is a GaAlAs LED as one of the LEDs, and since such LED exhibits high intensity, they are arrayed to provide an LED array which has been utilized as a light source for LED printer.
FIG. 1 is a plan view showing an example of conventional LEEDs, and FIG. 2 is a plan view showing another example of conventional LEDs.
LED array has a fine structure, and it is classified into two types. One of them is peripheral electrode type LED array (FIG. 1) wherein an electrode (peripheral electrode) 30 is disposed on a side (peripheral part) of a light take-out region 20 in a light emitting dot 10, and the other type is a central electrode type LED array (FIG. 2) wherein an electrode (central electrode) 31 is disposed at an central portion of a light take-out region 21 in a light emitting dot 11. In either of the LEDs, the electrode 30 or 31 is disposed through a contact layer 40 or 41 for electrode, and the electrode 30 or 31 is connected to a metal interconnection 50 or 51.
In the peripheral electrode type LED shown in FIG. 1, since the peripheral electrode 30 is disposed on a peripheral portion of the light take-out region 20, it is very difficult to uniformly expand an input current over the whole area of the light take-out region 20. As a result, the light which can be taken out from the light take-out region 20 decreases with increase in a distance from the peripheral electrode 30, because the light output decreases, resulting in unevenness of light output in the light take-out region 20, besides, high light output cannot be obtained in this case. Furthermore, there is a possibility of variations in a mode of expanding an input current depending upon a slight difference in crystallizability of each LED, so that variations of each LED in an LED array become also remarkable.
Moreover, since distribution of light intensity is biased toward the peripheral electrode 30, there has been such a problem that a light emitting region effective for printing of an LED printer is not arranged horizontally in a straight line, but staggered, even in the light take-out regions 20 of adjacent LEDs are arranged horizontally in a straight line.
On the other hand, in the central electrode type LED shown in FIG. 2, since the central electrode 31 is positioned in a central portion of the light take-out region 21, this type of LEDs can overcome the above-mentioned disadvantage involved in peripheral electrode type LEDs. In other words, the input current can be extended over the whole area of the light take-out region 21 in the central electrode type LEDs.
However, in the case where a finer structure of LED is required as, for example, in a high density LED array of 600 DPI or more, when a central electrode type structure is used, a size of the central electrode cannot be reduced further because of assuring reliability. For this reason, a ratio of LED covered by electrode (hereinafter referred to as "electrode covering ratio") increases, so that it becomes difficult to obtain an LED of high output. In addition, as a result of providing the remarkably narrow light take-out region 21, a problem arises also in a shape of light emitting spot.